A radioimmunoassay for porcine relaxin (NIH-RP-1) has been developed. The specificity of this assay for ovarian, uterine and placental relaxins in plasma and tissue extracts is being studied by the use of Sephadex column chromatography in conjunction with the assay. Studies of the physiological role of relaxin in the reproductive processes of the male and female are in progress in the sheep and human. The RIA is also being used to monitor the purification of the crude relaxin (NIH-RP-1) for subsequent use in the assay and for amino-acid analysis and sequence determination using a Beckman Sequenator.